12 research outputs found
Discreteness effects in cosmological N-body simulations
An estimate of the convergence radius of a simulated CDM halo is obtained
under the assumption that the peak phase-space density in the system is set by
discreteness effects that operate prior to relaxation. The predicted
convergence radii are approximately a factor 2 larger than those estimated for
numerical convergence studies.
A toy model is used to study the formation of sheets of the cosmic web, from
which DM haloes form later. This model demonstrates the interplay between phase
mixing and violent relaxation that must also be characteristic of spherical
collapse. In the limit that sheets contain arbitrarily many particles, it seems
that power-law profiles are established in both distance and energy. When only
a finite number of particles is employed, relaxation is prematurely terminated
and the power laws are broken. In a given simulation, the sheets with the
highest peak phase-space densities are those that form from the longest waves.
Hence simulations with little small-scale power are expected to form the
cuspiest haloes.Comment: 10 pages submitted to MNRA
Topology of non-linear structure in the 2dF Galaxy Redshift Survey
We study the evolution of non-linear structure as a function of scale in
samples from the 2dF Galaxy Redshift Survey, constituting over 221 000 galaxies
at a median redshift of z=0.11. The two flux-limited galaxy samples, located
near the southern galactic pole and the galactic equator, are smoothed with
Gaussian filters of width ranging from 5 to 8 Mpc/h to produce a continuous
galaxy density field. The topological genus statistic is used to measure the
relative abundance of overdense clusters to void regions at each scale; these
results are compared to the predictions of analytic theory, in the form of the
genus statistic for i) the linear regime case of a Gaussian random field; and
ii) a first-order perturbative expansion of the weakly non-linear evolved
field. The measurements demonstrate a statistically significant detection of an
asymmetry in the genus statistic between regions corresponding to low- and
high-density volumes of the universe. We attribute the asymmetry to the
non-linear effects of gravitational evolution and biased galaxy formation, and
demonstrate that these effects evolve as a function of scale. We find that
neither analytic prescription satisfactorily reproduces the measurements,
though the weakly non-linear theory yields substantially better results in some
cases, and we discuss the potential explanations for this result.Comment: 13 pages, matching proof to be published in MNRAS; new version adds
reference and corrects figure
The Shape of Galaxy Cluster Dark Matter Haloes: Systematics of Its Imprint on Cluster Gas, and Comparison to Observations
(Abridged) We study predictions for galaxy cluster observables that can test
the statistics of dark matter halo shapes expected in a flat LCDM universe. We
present a simple analytical model for the prediction of cluster-scale X-ray
observations, approximating clusters as isothermal systems in hydrostatic
equilibrium, and dark matter haloes as ellipsoids with uniform axial ratios. We
test the model against high-resolution, hydrodynamic cluster simulations to
gauge its reliability. We find that this simple prescription does a good job of
predicting the distribution of cluster X-ray ellipticities compared to the
simulations as long as one focuses on cluster regions that are less sensitive
to recent mergers. Based on this simple model, the distribution of cluster-size
halo shapes expected in the concordance LCDM cosmology implies an X-ray
ellipticity distribution with a mean of 0.32 +- 0.01 and a scatter of 0.14 +-
0.01 for the mass range (1-4)x10^{14} Msun/h. We find it important to include
the mass dependence of halo shape to make comparisons to observational samples
that contain many, very massive clusters. We analyse the systematics of four
observational samples of cluster ellipticities and find that our results are
statistically compatible with observations. In particular, we find remarkably
good agreement between two recent ROSAT samples and LCDM predictions that DO
NOT include gas cooling. We also test how well our analytical model can predict
Sunyaev-Zel'dovich decrement maps and find that it is less successful although
still useful; the model does not perform as well as a function of flux level in
this case because of the changing triaxiality of dark matter haloes as a
function of radial distance. Both this effect and the changing alignment of
isodensity shells of dark matter haloes leave an imprint on cluster gas...Comment: 16 pages, 9 figures; corrected typo (no result affected) submitted to
MNRA
Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 2. Lake, Marine, and Terrestrial Sediments
Part 1 of this study investigated evidence of biomass burning in global ice records, and here we continue to test the
hypothesis that an impact event at the Younger Dryas boundary (YDB) caused an anomalously intense episode of
biomass burning at ∼12.8 ka on a multicontinental scale (North and South America, Europe, and Asia). Quantitative
analyses of charcoal and soot records from 152 lakes, marine cores, and terrestrial sequences reveal a major peak in
biomass burning at the Younger Dryas (YD) onset that appears to be the highest during the latest Quaternary. For the
Cretaceous-Tertiary boundary (K-Pg) impact event, concentrations of soot were previously utilized to estimate the
global amount of biomass burned, and similar measurements suggest that wildfires at the YD onset rapidly consumed
∼10 million km2 of Earth’s surface, or ∼9% of Earth’s biomass, considerably more than for the K-Pg impact. Bayesian
analyses and age regressions demonstrate that ages for YDB peaks in charcoal and soot across four continents are
synchronous with the ages of an abundance peak in platinum in the Greenland Ice Sheet Project 2 (GISP2) ice core and
of the YDB impact event (12,835–12,735 cal BP). Thus, existing evidence indicates that the YDB impact event caused
an anomalously large episode of biomass burning, resulting in extensive atmospheric soot/dust loading that triggered
an “impact winter.” This, in turn, triggered abrupt YD cooling and other climate changes, reinforced by climatic
feedback mechanisms, including Arctic sea ice expansion, rerouting of North American continental runoff, and subsequent ocean circulation changes
Multiplatform Analysis of 12 Cancer Types Reveals Molecular Classification within and across Tissues of Origin
Recent genomic analyses of pathologically-defined tumor types identify “within-a-tissue” disease subtypes. However, the extent to which genomic signatures are shared across tissues is still unclear. We performed an integrative analysis using five genome-wide platforms and one proteomic platform on 3,527 specimens from 12 cancer types, revealing a unified classification into 11 major subtypes. Five subtypes were nearly identical to their tissue-of-origin counterparts, but several distinct cancer types were found to converge into common subtypes. Lung squamous, head & neck, and a subset of bladder cancers coalesced into one subtype typified by TP53 alterations, TP63 amplifications, and high expression of immune and proliferation pathway genes. Of note, bladder cancers split into three pan-cancer subtypes. The multi-platform classification, while correlated with tissue-of-origin, provides independent information for predicting clinical outcomes. All datasets are available for data-mining from a unified resource to support further biological discoveries and insights into novel therapeutic strategies